1. Field of Invention
The present invention relates to a guide plate, surface light source device of side light type and liquid crystal display, in particular, to a guide plate having improved emission characteristics, surface light source device of side light type employing the improved guide plate and liquid crystal display employing the surface light source device for illuminating a liquid crystal display panel.
2. Related Arts
Surface light source devices of side light type broadly employ as guiding-and-emitting means a guide plate (plate-like light guide) of a type such that light is introduced from a side end face provided by a minor face and is emitted from an emission face provided by a major face, because a thin-structured arrangement is realized easily by disposing a primary light source beside the guide.
And surface light source devices of side light type employing such a guide plate are applied to, for example, liquid crystal displays for lighting of liquid crystal display panel.
In the present specification, a side end face for introducing light is called xe2x80x9cincidence end facexe2x80x9d and another major face, which is opposite with the emission face, is called xe2x80x9cback facexe2x80x9d.
On lightening of a primary light source incorporated in a surface light source device of side light type, illumination light is introduced into a guide plate through an incidence end face. Thus introduced illumination light propagates in the guide plate, with light being emitted meanwhile little by little from an emission face of the guide plate, for example, toward a liquid crystal display panel.
Guide plates are classified into two types depending on cross section shape, one having an approximately uniform thickness, the other having thickness tending to reduce according to distance from a side end face. In general, the latter gives a higher emission efficiency as compared with the former.
In almost every case, surface light source devices of side light type are desired to output illumination light directed a frontal direction for the main. Known are two methods of modifying emission directivity to promote frontal outputting. According to one of the methods, a prism sheet is disposed along the emission face. And according to the other, a great number of projection rows are formed on a back face or emission face of the guide plate.
Usually, the former is applied for modifying directivity regarding in a plane perpendicular to the incidence end face. It is noted that a xe2x80x9cboth-prism-face prism sheetxe2x80x9d formed with prism rows running vertically to each other on both faces further enables directivity to be modified regarding in a plane parallel to the incidence end face.
On the other hand, the latter is applied for modifying directivity regarding in a plane parallel to the incidence end face. In this case, the projection rows are formed as to extend in an approximately vertical direction with respect to the incidence end face. Individual projection row includes a pair of slopes to form a triangle-like profile.
However, if the latter is applied to a conventional surface light source device of side light type, the device is subject to problem that abnormal emission (reflective appearance) tends to be brought in a part of guide plate. This abnormal emission appears on an emission face like some bright lines near to and along an incidence end face. So-called xe2x80x9creflective appearance of edgesxe2x80x9d is regarded as being causative of such abnormal emission.
In other words, upper and lower edges of an incidence end face of a guide plate are apt to be strongly irradiated by illumination light, often leading to a phenomenon such that the edge irradiation produces beams emerging as bright lines which can be observed from the outside of the emission face. Since illumination light that travels by way of an edge propagates within the guide plate so as to go and come back between the emission and back faces and some emission occurs at every chance of incidence to the emission face, not a single bright lines are observed.
It is noted that reflective appearance similar to the above can be caused by illumination light that travels by way of a lateral side edge of the guide plate. Such reflective appearance caused in connection with a lateral side edge tends to emerge as bright lines along lateral margins of the guide plate.
Abnormal emission caused by the above-described reflective appearance tends to be more striking if projection rows with triangle-like cross section are formed on a major face (emission face or back face) of the guide plate, as compared with a case where they are not formed (i.e. flat face). It may be surmised that this difference is due to that the projection rows cause illumination light travelling by way of an edge to be converged and emitted toward a frontal direction.
Needless to say, abnormal emission caused by reflective appearance reduces illumination quality of a surface light source device of side light type. Besides, if the device is applied to lighting of a liquid crystal display panel of a liquid crystal display, the display is subject to reduction in display quality.
The present invention is proposed under the above-described background and an object of the invention is to provide a guide plate which is improved so that abnormal emission caused by reflective appearance of edges is reduced. Another object of the invention is to provide a surface light source device of side light type having an improved illumination quality by the use of the improved guide plate, and still another object of the invention is to provide a liquid crystal display having an improved display quality by the use of the improved surface light source device of side light type for backlighting of a liquid crystal display panel.
The present invention can be applied to a guide plate which comprises a minor face to provide an incidence end face for introducing light and major faces to provide an emission face for emitting light which has been introduced through said incidence end face and to provide a back face which is opposite with said emission face.
One of the emission face and back face of the guide plate improved according to the present invention is provided with a great number of projections extending in a direction approximately vertical to said incidence end face. These projections include arc-like-profiled projections located within a first predetermined range near to said incidence end face. This causes light propagating by way of an edge to be emitted from the emission face after being subject to direction expansion involved by inner reflection at a projection. As a result, abnormal emission like bright lines hardly to appears on the emission face. In other words, abnormal emission caused by reflective appearance is reduced.
The projections are preferably configurated so that projection profile transits gradually from an arc-like-shape to a triangle-like shape according to distance from the incidence end face in order to avoid affection of sharp profile changing.
It is also preferable that the projections further include arc-like-profiled projections located within a second predetermined range along a flank face of said guide plate. This prevents abnormal emission which could emerge around the lateral side edges. A typically employed guide plate is made of a scattering-and-guiding material provided with uniform scattering power inside. A transparent guide plate may be employed.
The present invention can be applied to a surface light source device of side light type including a primary light source an a guide plate which comprises a minor face to provide an incidence end face for introducing light supplied by the primary light source and major faces to provide an emission face for emitting light which has been introduced through said incidence end face and to provide a back face which is opposite with said emission face.
According to the present invention, the guide plate is the above-mentioned improved guide plate. This gives an improved quality to the output illumination light.
The present invention is further applied to a liquid crystal display including a liquid crystal display panel and a surface light source device of side light type for illuminating the panel.
The surface light source device of side light type includes a primary light source an a guide plate which comprises a minor face to provide an incidence end face for introducing light supplied by the primary light source and major faces to provide an emission face for emitting light which has been introduced through said incidence end face and to provide a back face which is opposite with said emission face. The guide plate is the above-mentioned improved guide plate. This brings an improved display quality.
The above-described features and other features of the present invention will be understood in more details by the following description and the accompanied drawings to be referred.